This invention relates generally to the field of integrated optics chips or devices and more particularly to the field of multifunction integrated optics chips such as those having integrated optic components formed in lithium niobate (LiNbO3) substrates. Integrated optics components formed on such chips include waveguides that may be arranged to function such as polarizers, optical couplers and phase modulators. Multiple functions may be incorporated on a single device, which eliminates losses and errors associated with interfacing separate devices. This invention is particularly directed to methods and apparatus for reducing polarization non-reciprocity errors in a multifunction integrated optics chip as a result of scattered wave propagations. which can cross couple into the fibers which are pigtailed to the optical waveguides.
Multifunctional Integrated Optical Chips (MIOC""s) are usually fabricated in large numbers on three to four inch wafers of lithium niobate (LiNbO3) using conventional photomasks, vacuum deposition, chemical baths, proton exchange, diffusion, and etching techniques to form large numbers of identical components at low cost and with high reliability. MIOC""s capable of performing the aforementioned functions are used in fabricating middle and high accuracy fiber optic gyroscopes (FOG""s) or rotation sensors. The FOG uses the Sagnac effect to measure rates of rotation about an axis perpendicular to a coil of optical fiber. MIOC""s may also be used in forming other fiber optic sensors such as hydrophones and geophones that rely on the principles of the Mach-Zehnder or Michelson interferometer.
A fiber optic gyroscope includes means for introducing counterpropagating waves from an optical signal source into an optical fiber coil. Rotation of the coil about an axis perpendicular to the plane of the coil produces a phase difference between the clockwise and counter-clockwise wave via the Sagnac effect. The phase shift occurs because waves that traverse the coil in the direction of the rotation have a longer transit time through the optical fiber coil than waves that traverse the coil in the opposite direction. The waves are combined after propagating through the coil. This combination of waves produces an interference pattern that may be processed to determine the rate of rotation. Techniques for determining the rotation rate are well-known in the art.
It is common practice to form a FOG to include a multifunctional integrated optics chip (MIOC) between the optical signal source and the optical fiber coil, which is typically formed of polarization maintaining fiber. The MIOC typically includes a plurality of optical waveguides arranged to form a Y-junction. The base of the Y-junction is connected to the optical signal source while the arms of the Y-junction are interfaced with ends of the optical fiber coil. Optical signals input to the multifunctional integrated optics chip divide at the Y-junction to form optical signals that are input to the ends of the optical fiber coil as the counterpropagating waves. After propagating through the coil, the waves enter the optical waveguides that form the arms of the Y-junction. The waves then combine in the Y-junction and are output from the base of the Y-junction to an optical fiber. The combined waves are guided to a photodetector that produces an electrical signal that is processed to determine the rotation rate.
The desired condition in a fiber optic rotation sensor is the transverse electric (TE) mode propagating in the optical fiber coil and in the optical waveguides without added path lengths. Propagation of transverse magnetic (TM) modes and TE modes having added path lengths are undesired conditions. Error sources such as polarization cross coupling, which adds a phase shift (or polarization non-reciprocity, PNR, which is associated with always having two polarization components possible in the fiber at all times), manifest themselves as additional optical path differences in direct competition with the Sagnac effect. These error sources cause phase bias and amplitude bias errors when they are modulated at the frequency used by the phase modulators in the MIOC. The bias component in the fiber optic rotation sensor due to polarization cross coupling is inversely proportional to the square root of the absolute value of the polarization extinction ratio. Extinction ratio is defined as ten times the logarithm of the ratio of the undesired power (the power of the undesired mode) to the desired power (the power of the desired mode) of the polarization modes expressed in decibels. Minimizing cross coupling (maximizing the absolute value of the extinction ratio) in the MIOC reduces this type of bias error.
As further background, integrated optics chips (IOC""s), such as those disclosed herein, may be formed using processes and steps similar to some of those disclosed in U.S. Pat. No. 5,193,136, which issued to Chin L. Chang et al. on Mar. 9, 1993 for PROCESS FOR MAKING MULTIFUNCTION INTEGRATED OPTICS CHIPS HAVING HIGH ELECTRO-OPTIC COEFFICIENTS; U.S. Pat. No. 5,046,808, which issued to Chin L. Chang on Sep. 10, 1991 for INTEGRATED OPTICS CHIP AND METHOD OF CONNECTING OPTICAL FIBER THERETO; U.S. Pat. No. 5,393,371, which issued to Chin L. Chang et al. on Feb. 28, 1995 for INTEGRATED OPTICS CHIPS AND LASER ABLATION METHODS FOR ATTACHMENT OF OPTICAL FIBERS THERETO FOR LiNbO3 SUBSTRATES; U.S. Pat. No. 5,442,719, which issued to Chin L. Chang et al. on Aug. 15, 1995 for ELECTRO-OPTIC WAVEGUIDES AND PHASE MODULATORS AND METHODS FOR MAKING THEM; and U.S. Pat. No. 4,976,506, which issued to George A. Pavlath on Dec. 11, 1990 for METHODS FOR RUGGED ATTACHMENT OF FIBERS TO INTEGRATED OPTICS CHIPS AND PRODUCT THEREOF.
Each of the foregoing patents is assigned to Litton Systems, Inc. of Woodland Hills, Calif. Each of the foregoing patents cited above is incorporated herein by reference for the purpose of providing those skilled in the art with background information on how integrated optics chips (IOCs) or multifunctional integrated optical chips are made.
If the gyro bias is significantly reduced, there is the potential to reduce the fiber costs by replacing polarization maintaining fiber with less expensive single mode fiber, or using a shorter length of polarization maintaining fiber than is presently used.
An integrated optics chip according to the present invention comprises an optical waveguide network formed in a substrate of an electrooptically active material. The optical waveguide has input and output facets where optical signals may be input to and output from the integrated optics chip. At least one lateral trench is formed in the substrate. The lateral trench is arranged to prevent light rays incident thereon from inside the substrate from propagating to the output facet.
The lateral trench may be formed to extend toward the surface of the substrate where the optical waveguide network is formed. The trench is formed as a slot that makes an acute angle with first surface. The trench may be formed in a surface that is either parallel or perpendicular to the plane of the optical waveguide network.
The lateral trench preferably extends completely across the width of the substrate and extends into the substrate to a depth that is about 75% to 95% of the substrate depth. The endpoint of the lateral trench preferably is located at a distance along the length of the substrate about 10% to 30% of the length of the substrate from an end thereof.
The integrated optics chip according to the present invention may include a second trench that is formed in the substrate to be symmetrical with the first trench.
A light absorbing material may be placed in the lateral trenches.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.